1. Field of the Invention
The present invention relates generally to wireless communications and in particular to cellular telephony.
2. Description of the Related Art
Frequently wireless network designers and equipment suppliers are tasked with providing reliability estimates for coverage of cellular systems. Service providers want to know how reliable their coverage is and want to be able to offer more reliable service to their customers.
Two standard ways to express cell reliability are illustrated in FIG. 1. They are contour (or cell edge) reliability as illustrated in FIG 1a, which gives the reliability of a user who travels the contour of the cell edge; and cell area reliability as illustrated in FIG. 1b, which gives the reliability of a user who may be anywhere within the confines of the entire cell boundary--not just on the cell edge. Both standards are often specified by a user or service provider.
The coverage probabilities requested by a user typically range from 70% to 99%. For example, public safety users generally require 95% contour reliability which corresponds to no more than a 5% failure within the entire coverage area. Commercial users may be satisfied with 70% contour reliability which translates to a 90% reliability across the entire cell area.
Previous designers were required to perform extensive RF measurements to establish cell boundary and reliability figures of merit. Typically, field engineers would drive the cell area collecting data with a Received Signal Strength Indicator (RSSI) receiver for measuring the received signal strength from a central transmitter. The exact location of the receiver is known through Global Positioning System (GPS) receivers attached to the RSSI receiver. The previous systems and methods for estimating cell coverage and reliability used hundreds and thousands of data points and still came up with estimates that ranged within plus or minus ten percent of actual (later) measurements were reluctantly considered acceptable.
The well known Hata method for estimating path loss is inherently error-prone and is valid over only a very limited range under certain specific conditions. A different correction factor is needed each and every time the method is used. The correction factor may be determined for either a small, medium or large sized city and does not take into account features of the particular cell under study. The unreliability is magnified if used to calculate cell radius and/or reliability.
There is accordingly a need for a new method and apparatus for reliably measuring and predicting the boundary of a cell and for the coverage within the cell in order to solve or ameliorate one or more of the above-described problems.